A thick isolating oxide layer is conventionally grown into the surface of a silicon substrate between locations where Insulated Gate Field Effect Transistors (IGFETs) are formed to electrically isolate adjacent IGFETs from one another. Generally such isolating oxide layers are formed by a sequence of steps known as the LOCal Oxidation of Silicon (LOCOS) process. U.S. Pat. Nos. 3,752,711 and 3,970,486, and Canadian Patents Nos. 826,343 and 920,284 describe LOCOS processes.
In the most commonly used LOCOS process, a thin oxide buffer layer is grown over the entire surface of a silicon substrate. A thick nitride layer is then deposited over the entire oxide buffer layer. The nitride layer is patterned photolithographically to create openings in the nitride layer where a thick isolating oxide is desired. Channel stop impurities may then be implanted through the openings, and silicon beneath the openings is thermally oxidized to grow a thick oxide into the substrate at the openings. The remaining portions of the nitride and buffer oxide layers are then removed, and IGFETs are formed in and on the exposed substrate surface.
In the above process, the thin buffer oxide layer is provided to relieve stresses which would otherwise be applied to the surface of the silicon substrate if the thick nitride were grown or deposited directly on the silicon surface. The thin buffer oxide layer also provides a convenient etch stop during photolithographic definition of the nitride layer. Unfortunately, the thin buffer oxide layer permits lateral growth of the thick isolating oxide layer under the nitride layer at edges of the openings through the nitride layer. This lateral growth of the isolating oxide layer results in "bird's beak" formations which consume valuable area on the surface of the substrate which could otherwise be used for IGFET devices, thereby limiting the maximum device density. The "bird's beak" formations also aggravate "narrow channel effects" which degrade the performance of the IGFET devices.